TOBA is a-swingin’, looking for gravity waves

Scientists in Japan present the first results from their test version of a …

A few months ago, we reported on a theoretical paper that discussed the potential advantages of a gravity wave detector based on a torsion bar, which the creators called TOBA. In the intervening time, the team has not been idle, as they have a small-scale test bar up and running. Deep in the night of August 15, 2009, they performed a test run to look for gravity waves—not gravity induced pressure fluctuations in Earth's atmospheric pressure, but stretching space-time.

The good thing about the TOBA experiment is that it fills an important spectral gap in the current generation of gravity wave detectors. Cosmological and astronomical observations can be used to look for extremely low frequency (10-6Hz) gravitational waves, while the laser interferometer detector (LIGO), and other Earth-bound instruments are used to look for gravity waves with frequencies in the 100Hz plus range. TOBA is designed to operate in the 0.1-1Hz range.

This is important because of something called the cosmic gravitational wave background. You may have heard of cosmic background radiation. This radiation is the oldest in the Universe. It originates in that time interval when the hot dense Universe went through a phase transition from a plasma (consisting of unbound electrons and ions) to neutral atomic species. Before this time, radiation was scattered an awful lot, meaning that its history was rapidly lost. After the Universe became neutral, the scattering was very much reduced, allowing this light to retain its history. The remnant that we measure is the unscattered light from that transition but, optically, that moment in time is like a wall that we simply cannot see through.

So, all the physics of the Universe from times before the transition must be tested rather indirectly, based on what the physics implies about the phase transition. That doesn't mean it's all guesswork, but more direct observational data would always be welcome.

This is where gravitational waves can play an important role. Gravity waves don't care about electromagnetic charge, so this phase transition is unimportant as far as they are concerned. Furthermore, the early Universe is thought to have rung like a bell, with space-time stretching and contracting rhythmically. The frequency and directionality of these waves will tell us about the symmetry properties and, as a result, the physics of the early Universe. These waves are spread across the low frequency end of the gravity wave spectrum, including the range covered by TOBA.

Now that we know why it's exciting, let's take a look at TOBA itself. The little TOBA is a torsion bar with some cleverness attached to it. It's not suspended by a wire; instead, the Japanese research group used a property of a particular class of superconductor to hold the torsion bar suspended in vacuum. When a magnetic field penetrates a superconductor, it gets pinned to a specific location. By attaching a magnet to the center point of the torsion bar and hanging the torsion bar below a superconductor, the magnetic field lines get held in place, creating a suspension system that has very little resistance.

The result is that, when the torsion bar is set in motion, it stays in motion for a long time. Another consequence of this low resistance suspension is that the resonant frequency of the torsion bar is very low—around 5mHz in this case—making it very sensitive to low frequency disturbances in the gravitational field.

As with all things, I suspect the experiment has been operational for some time now. But all the early work would have been making sure that the performance was as expected. After all the additional sources of noise had been expunged, the research group took 12 hours of preexisting data, selected the least noisy sections, and analyzed them for the presence of gravity waves. End result: they didn't find any. I suspect they would have been shocked if they had.

This paper isn't really about the gravity wave data; instead, it is about an instrument that, in its first stage of development, is performing as expected. What is next? The researchers plan to move the experiment to the bottom of a mine, so that the environment is a bit quieter. They plan to improve the vacuum conditions, so that the intrinsic instrumental noise is reduced. They also plan to add a second torsion bar at right angles to the first. This second bar will allow the researchers to eliminate many local sources of vibrational noise, increasing their sensitivity further.

Finally, should all of that work as expected, they get to build the big version of the experiment. Clearly there are many more years of work and several excellent research projects waiting for students here. And I look forward to seeing them report their progress.

So if they are able to actually measure a gravity wave, does that mean in the future we will be able to generate them?

Also what's with the sperm image?

The sperm image is related to what is going to happen between William and Kate in a few hours - from a biological perspective- as it relates to their gravitational field. Kate's oven is gonna be cranking out some eggies with a highly resonant frequency

I get that it's incredibly difficult to detect the things, and I get that most of our most cherished theories say they have to be there, but what would the consequences of not finding any be? Assuming (as always) there's high confidence that the instruments would be able to detect them if they existed?

So if they are able to actually measure a gravity wave, does that mean in the future we will be able to generate them?

You generate a gravitational wave every time you move. As does each atom of the air you breathe. Since the size of a wave is proportional to the mass of the object being moved and we're still unable to detect waves from what theory says is the brightest known source (the crab nebula pulsar) I wouldn't hold my breath on any practical transmitter/receiver pair systems being created.

A few years ago Einstien@Home posted a few posters they submitted to a conference. Their estimate of crab nebula pulsars signal strength (with +- 2 order of magnitude error bars) was sitting right on LIGO's theoretical detection threshold. However the 54(?) hz signal from it was very near to a huge 60hz noise spike (separate graphs so it wasn't clear if the signal would be swamped or not). The final challenge is that the total amount of data produced is far larger than can be analyzed, and their current processing regime requires a signal roughly one order of magnitude higher than the theoretical limit to guarantee detection (the old method needed 2 orders of magnitude), the falloff for detection below that appeared roughly linear on the (log scale?) graph.

The Advanced LIGO upgrades should allow for detecting several known pulsars, the ETA for completing the hardware is 2014. Until then E@H is mostly looking for unknown pulsars close to the Earth. (LIGO is also looking for transient signals like neutron star/black hole mergers; but they don't send that data out for external processing.)

I get that it's incredibly difficult to detect the things, and I get that most of our most cherished theories say they have to be there, but what would the consequences of not finding any be? Assuming (as always) there's high confidence that the instruments would be able to detect them if they existed?

Noone's detected them directly, we have strong indirect evidence from the decay of orbits in several binary neutron stars. If GWs don't exist there's a huge hole in general relativity.

Lack of gravity waves would substantiate the recently posted here at Ars article that Gravity is not a force.

I however, believe in gravity waves, but also believe that they are practically impossible to find. The reason is mass is rarely created or destroyed (converted to massless energy) and everything on galactic scales is point-mass math appropriate. So even if you have co-orbiting black holes, after light years the waves are indistinguishable from each other. I expect it to be measurable nearer to the event horizon.

Finally, my problem with this specific experiment is that superconductors are known to have a gravity shielding effect, which is likely to be dominating in the local test.

As with a lot of NI articles, the concepts are at least explained in such a way that none of this feels over my head - but it's hard to shake the feeling that the really exciting stuff is just above my level of comprehension.

As with a lot of NI articles, the concepts are at least explained in such a way that none of this feels over my head - but it's hard to shake the feeling that the really exciting stuff is just above my level of comprehension.

Not a bad way to enter the weekend.

DanNeely wrote:

If GWs don't exist there's a huge hole in general relativity.

In a way, isn't that more exciting?

Exciting in that learning is fun, and annoying in that it'll set us back a few more centuries of research.

Wouldn't this be a good item to put in space? Also curious about how they shield the guts from any measurement electronics or otherwise cancel their effect.

LISA is a proposal for a space based interferometer; but the current nasa funding situation makes it unlikely to be built.

Ground based systems are heavily dependent on vibration damping and have all the hardware suspended on cables (resonant/harmonic frequencies of them represent major noise spikes). Pulsar spindown and binary in-spiral are both CW signals modulated by the earths rotation and orbit around the sun, so checking that the signal remains across months of data is an import part of the detection process. Transient signals (eg NS/BH mergers) don't have that option, but can be cross checked against multiple locations (LIGO has 2 4km, and 1 2km interferometer in two locations, VIRGO operates a 3km unit at a 3rd); without the time integration however they'd need to be significantly higher than CW signals to be detected. LIGO's sensitivity for the latter is, IIRC ~1 part in 10^28, or the diameter of a hydrogen atom vs the Earth-Saturn distance.

Lack of gravity waves would substantiate the recently posted here at Ars article that Gravity is not a force.

I however, believe in gravity waves, but also believe that they are practically impossible to find. The reason is mass is rarely created or destroyed (converted to massless energy) and everything on galactic scales is point-mass math appropriate. So even if you have co-orbiting black holes, after light years the waves are indistinguishable from each other. I expect it to be measurable nearer to the event horizon.

Finally, my problem with this specific experiment is that superconductors are known to have a gravity shielding effect, which is likely to be dominating in the local test.

1) Even if gravity is "not a force" in the same fundamental sense as the others, there would still be gravitational waves. Gravitational waves are not caused by some special feature of "forces" or "gravity", but are a direct consequence of Lorentz invariance under general relativity. As long as physical interactions have a maximum propagation speed, then there would still be something like a gravity wave associated with any gravity "like" thing, whether a fundamental force or not.

2) Mass is never created or destroyed. Mass (the thing associated with gravity) is 100% conserved, at all times. Matter (fermions) can be created or destroyed, but even collections of photons can have mass. During nuclear reactions, the total mass doesn't change, a little of it simply escapes with the gamma-rays.

3) Your "point-mass" idea doesn't work. It doesn't matter if there is virtually no angular separation at galactic scales; angular separation is not what we are trying to measure. The gravity waves produced by a rotating binary spread out as a spiral pattern, which at extreme distances will look like a plane wave, which does not "disappear" with distance because of lack of angular separation.

4) superconductors are not "known to have a gravitational shielding effect". There is one Russian anti-gravity advocate who has made some claims about that, but it has not been replicated, and "the consensus view of physicists is that gravitational shielding does not exist" (Wikipedia). If you want to play science, you have to play by the rules, and that means that one person's unreplicated anti-gravity results are not a "known effect" to real scientists.

As with a lot of NI articles, the concepts are at least explained in such a way that none of this feels over my head - but it's hard to shake the feeling that the really exciting stuff is just above my level of comprehension.

Not a bad way to enter the weekend.

DanNeely wrote:

If GWs don't exist there's a huge hole in general relativity.

In a way, isn't that more exciting?

Exciting in that learning is fun, and annoying in that it'll set us back a few more centuries of research.

This. I'm far more keen on actually getting somewhere. The sooner we can develop a complete, unabridged understanding of physics, the sooner we can develop the technology to harness said physics. I want FTL, dammit! That's not something we're going to achieve until we understand, and can manipulate gravity.

1) Even if gravity is "not a force" in the same fundamental sense as the others, there would still be gravitational waves. Gravitational waves are not caused by some special feature of "forces" or "gravity", but are a direct consequence of Lorentz invariance under general relativity. As long as physical interactions have a maximum propagation speed, then there would still be something like a gravity wave associated with any gravity "like" thing, whether a fundamental force or not.

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Well if gravity is re result of a projection as contended in the paper, then it would not be necessary to have a propagation time. Without a propagation time, you'd never have a wave.

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2) Mass is never created or destroyed. Mass (the thing associated with gravity) is 100% conserved, at all times. Matter (fermions) can be created or destroyed, but even collections of photons can have mass. During nuclear reactions, the total mass doesn't change, a little of it simply escapes with the gamma-rays.

I'm right there with you. I was referring to if you're looking for gravity waves from a super nova, you are unlikely to find one. There is some mass lost, but most of it - what doesn't go to gamma rays - is just converted.

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3) Your "point-mass" idea doesn't work. It doesn't matter if there is virtually no angular separation at galactic scales; angular separation is not what we are trying to measure. The gravity waves produced by a rotating binary spread out as a spiral pattern, which at extreme distances will look like a plane wave, which does not "disappear" with distance because of lack of angular separation.

What I'm trying to say, is that spiral pattern isn't likely to exist. A binary black hole system has cores 3.2 light years apart, but the are 5 billion light years from earth. Given that gravity is inversely proportional to square of distance, I just don't see how you'd ever get a usable signal at this instance. And when you factor in local time dilation and other relativistic effects to the distance system, my brain starts to hurt.

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4) superconductors are not "known to have a gravitational shielding effect". There is one Russian anti-gravity advocate who has made some claims about that, but it has not been replicated, and "the consensus view of physicists is that gravitational shielding does not exist" (Wikipedia). If you want to play science, you have to play by the rules, and that means that one person's unreplicated anti-gravity results are not a "known effect" to real scientists.

Chris Lee / Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands.